Deicing systems for aircraft have typically used a water and glycol mixture which is sprayed on the aircraft while the aircraft is in the parking area awaiting clearance for takeoff. However, the aircraft is frequently required to wait for a long time and ice may build up again on the aircraft surfaces. The ethylene glycol used may not be sprayed in or around the aircraft engines and the glycol may be considered a hazardous or potentially hazardous material to the environment. Glycol is an expensive mixture and significant sums are spent every year by the airline industry for deicing fluid.
Deicing glycol mixtures have usually been a heated 50% solution of ethylene glycol with water. Deicing fluid is sprayed on the aircraft, usually only on its wing, tail and control surfaces, until a deicing truck operator believes the ice, snow or frost to be gone. Deicing may take up to 30 minutes and use several hundred gallons of ethylene glycol. The average cost at present day to deice a small to medium size commercial passenger airplane such as an MD-80, Boeing 727, 737 or 767 is $3,500-$4,000. A large jet such as a 747 with a 1/8" accumulation of clear ice may cost $10,000 in deicing fluid alone.
The pilot must also rely on the judgment and training of the deicing truck operator who may not be adequately trained or appreciate the degradation in coefficient of lift caused by ice alteration of the airfoil surface or the weight of the ice on the airframe. The pilots are without ice detector instruments to monitor wing or control surface icing. The only instruments available to the cockpit crew are engine inlet sensors which are effective against engine anti-icing. The aircraft is normally equipped with wing and tail leading edge anti-icing systems using hot air pulled from jet engine turbine stages but these can only be used while airborne or for only short periods while on the ground. Further, they tend not to be effective against accumulations of ice and are anti-icing systems rather than deicing systems.
Not only is the deicing process expensive and the pilot without effective instrumentation, but the glycol solution has been listed by the Environmental Protection Agency as a hazardous material. Glycol is a poisonous material and causes damage to wildlife. As a result, the EPA requires users to build containment basins and diversion structures to keep glycol out of watersheds. Nevertheless, containment structures are only partially effective, release into sewage treatment plants requires special processing and airborne spray remains difficult, if not impossible to contain. Yet, glycol deicing and anti-icing is still generally considered to be the most effective method of aircraft ice elimination and prevention.
Anti-icing uses a more viscous form of glycol as an anti-icing fluid, polyethylene glycol. This is sprayed onto the aircraft after ethylene glycol has deiced the aircraft. The anti-icing fluid is used only in small amounts and tends to stay on the aircraft surfaces until blown off during take-off. Because only comparatively small amounts are used, anti-icing fluid is not considered to be as problematical as use of deicing fluid.
The aircraft skin temperature may be approximately that as would be obtained during a hot summer afternoon in the U.S. southern latitudes, 180.degree. F. This procedure and temperature has been approved by the Federal Aviation Administrators as noninjurious to the aircraft. It does not cause the paint to blister, sealants to weaken, wiring insulation to char or fuel in wing tanks to dangerously vaporize.